As the cellular and wireless communications become more prevalent, new applications and terminals are designed to meet the growing demand of the market. The new 2.5G and 3G cellular systems such as GSM/EDGE/GPRS, WCDMA, TD-SCDMA and cdma2000 and 4G systems such as WiMax support a variety of data rates and applications. Apart from voice, new high data-rate applications such as video streaming and internet browsing are also supported by these systems. The support of data applications by such systems has lead to a variety of new terminals such as the PDAs and laptops with data cards or embedded cellular modems.
These new terminal types do not conform to a handset size and are not limited to handset power consumption, nor do they conform to the use case scenarios considered for handheld voice terminals. Therefore, a growing number of data cards and embedded modems are using well-known techniques such Antenna Space Diversity (ASD) in the receivers to improve the link performance. Further, more advanced multiple antenna techniques such as Transmit-Diversity (TD) and Multiple-Input Multiple-Output (MIMO) schemes are specified at the outset of the system design for the emerging 4G cellular systems, enabling these new systems to surpass the link speed and quality of the 3G systems. These new space-time diversity techniques provide gains in excess of 10 dB in the link-budget, improving data rates, indoor and outdoor Probability of Coverage (PoC) and the cell range and robustness.
Although these highly beneficial multiple antenna techniques (ASD, TD & MIMO) are being gradually introduced in the cellular systems, the current repeater architecture that employ a single receiver and a single transmitter antenna, often with some degree of directivity, cannot support communications using multiple antennas, diminishing the gain possible with such techniques.
FIG. 1 shows an ASD enabled Mobile Station (MS) in communications with the Base Transceiver Station (BTS) via a conventional repeater. The BTS does not use any Transmit Diversity (TD) scheme in this example. Although the link is bidirectional, only the downlink example is considered here for convenience. Even though the uplink may have different behavior than the downlink, the downlink discussions are also valid for the uplink of the system.
On the downlink, the MS antenna diversity will provide two signals with two different Signal-to-Noise Ratios (SNRs) to the diversity algorithm of the terminal unit. These two signals can then be combined by well known algorithms such Maximal-Ratio Combining (MRC) or Equal-Gain Combining (EGC) or other algorithms such as Selection Combining (SC) or Switched Combining (SWC). In either case, in average, a better final SNR is provided by the diversity scheme above the average SNR possible with a single antenna receiver.
The SNR improvement with diversity antennas is realized by combining or selecting signals received by the MS antennas (Ant 1 and 2). If the MS received signals were directly from the BTS, the only additive noise entering each antenna would have been Nir1 and Nir2 respectively. So each branch SNR will then be the ratio of the received signal (Sir) power to the noise power (Nir). However, in the case where the MS received signals are from a repeater, the repeater has already added a fixed and irreducible level of noise power (Nr) to the repeater transmitted signal (So), which is received by MS antennas. Therefore, regardless of the SNR gain of the space-diversity of the MS receiver, the single-antenna (i.e. no diversity) SNR of the repeater receiver sets the maximum performance limit achievable with the repeater in the link.
The repeater “original” received signal from BTS (Sio), enters the repeater antenna with a single SNR set by the additive thermal noise entering the receiver (Nio) and the effective repeater noise (Ne). As the repeater gain (G) is applied to the signal (Sio) and the repeater noise (Ni+Ne), repeater gain can NOT improve the “original” SNR of the single-antenna receiver. Therefore, the repeated signal (So), which is received by MS will at best have the repeater output SNR, which is worst than the repeater input SNR by the repeater Noise Figure (NF).
From the discussion above it is clear that a single antenna repeater can not provide the gain possible with the space-diversity at the mobile receiver. The loss of diversity gain is exacerbated by the directional antennas of the conventional repeaters, as the directionality of the antenna reduces or eliminates the multipath components of the received (or transmit) signal, further reducing the diversity gain at receiving end, be it at BTS or MS side.